From the video below, [Mike Rankin] has been working down the scale in terms of powering and sizing his ESP32 builds. He recently completed a project with an ESP32 Pico D4 and an OLED display that fits exactly on an AA battery holder, which he populated with a rechargeable 14550. Not satisfied with that form factor, he designed another board, this time barely larger than the LIR2450 rechargeable coin cell in its battery holder. In addition to the Pico D4, the board sports a USB charging and programming socket, a low drop-out (LDO) voltage regulator, an accelerometer, a tiny RGB LED, and a 96×16 OLED display. Rather than claim real estate for switches, [Mike] chose to add a pair of pads to the back of the board and use them as capacitive touch sensors. We found that bit very clever.

Sadly, the board doesn’t do much – yet – but that doesn’t mean we’re not impressed. And [Mike]’s no stranger to miniaturization projects, of course; last year’s Open Hardware Summit badge was his brainchild.

Rechargeable coin cell batteries are great for all your small projects. They look exactly like regular coin-cell batteries, but in a shocking turn of events you can recharge these little guys. They can put out a reasonable amount of current, and they’re small. Just what you need for your Arduino smart watch, or whatever else the kids are doing these days.

But if these batteries are rechargeable, you need a charger. That’s where [Jon]’s entry for the Hackaday Prize comes in handy. It’s a small, cheap charger for LIR2032 and other rechargeable batteries comes in. It’s barely larger than the battery itself, and it plugs right into a USB port. How this isn’t a product already, we’ll never know.

The circuit on this coin cell charger is built from an MCP73831, a nice single cell, lithium ion and lithium polymer charge management controller. In the standard, ‘I only need to read the first page of the datasheet’ configuration, this chip can put 500 mA into a battery. Standard rechargeable coin cells only have a capacity of 40 mAh, so you’ve got plenty of headroom at 1C.

The total cost for this project was under $8 for three boards, and a BOM cost of $2 for one. That’s fourteen bucks for three of them, if you know how to solder, compared to a standard, off-the-shelf charger for about $20. Building this is cheaper than buying the equivalent product. It’s unbelievable, but true.

In case you happen to have an ocean nearby, you’re probably familiar with its rising and falling tides. And if mudflat hiking is a thing in your area, you’re also aware of the importance of good timing and knowing when the water will be on its way back. Tide clocks will help you to be prepared, and they are a fun alternative to your usual clock projects. If you’re looking for a starting point, [rabbitcreek] put together an Arduino-based tide clock kit for educational purposes.

If you feel like you’re experiencing some déjà vu here, this indeed isn’t [rabbitcreek]’s first tide clock project. But unlike his prior stationary clock, he has now created a small and portable, coin-cell version to take with you out on the sea. And what shape would better fit than a 3D printed moon — unfortunately the current design doesn’t offer much waterproofing.

We’ve been tuned into coin cell designs lately given the coin cell challenge, so we were interested in [CNLohr]’s latest video about pushing the ESP8266 into the lowest-possible battery drain with coin cells. The result is a series of hacks, based on a reverse-engineered library and depends on a modified router, but that gets the power consumption down by more than a factor of ten!

Although the ESP8266 has a deep sleep mode that draws only 20 microamps or so, that isn’t as rosy as it seems. If you could go to sleep for a while, wake up for just a moment, send your data, and then go back to sleep, that might be one thing. But when you use conventional techniques, the device wakes up and has to do about ten seconds of work (at high power) to connect to a nearby access point. Then it can do what you want and go back to sleep. That ten-second hit is a killer on small batteries.

Since that’s all you can do with the standard libraries, the next step was to find [pvvx] who has reverse engineered a great deal of the libraries and provides a library with no WiFi capability. That’s a two-edged sword. The pro is you get a 30 ms startup from a deep sleep. The downside is — well — you don’t have WiFi.

At its heart is a modified Arduino Nano clone that draws a measured 608 nA from a CR2450N. From the specification of the cell he has calculated the 50 year maximum figure, as well as a possible 29 years for a CR2032 and 64 years for a CR2477. He does however note that this does not take self-discharge into account, but you can probably afford a new battery in a decade or so.

The Arduino clone carefully selected for its “P” version low-power processor has had its serial bridge IC removed to achieve this power consumption, as well as a voltage regulator and some discrete components. Interestingly he notes that the ATMega168P is even more frugal than its 328 cousin, so he’s used the former chip. A selection of internal flags are set for minimal power consumption, and the internal oscillator is selected to use as low a clock speed as possible. There is an Intersil ISL1208 low power RTC chip mounted on a piece of stripboard to provide the timing, and of course an LED to provide the essential birthday alert.

Here’s a fun entry into our coin cell challenge. The power source is the actuating force in [Frank]’s blinky LED Christmas tree, which takes advantage of the physical structure of coin cells and our old pal gravity to roll out some holiday cheer. Talk about forward voltage!

We love the concept, and the circuit couldn’t be more simple. A coin cell is released at the top of the tree and rolls down a series of angled foam board railings covered with 1/4″ copper tape. As the coin cell travels, the negative terminal shimmies along the face of the tree, which has corresponding ground rail tapes. There’s no microcontroller here—all that’s needed for blinks are breaks in the negative rail tape.

The challenging part of a project like this is the execution. Getting a coin cell to ride the rails without falling off required angle experimentation prior to and during the build. Now that it’s done, keeping the tree tilted back against the wall is key. [Frank] explored several options for returning the coin cell to the top using a camera motor and the gear assembly from an old inkjet, but for now, his six-year-old does the job without complaint. Check out his work ethic after the break.

This is it. This is the last weekend you’ll have to work on the most explosive battery-powered contest in recent memory. This is the Coin Cell Challenge, and it’s all ending this Monday. You have less than 48 hours to create the most amazing thing powered by a coin cell battery.

Joseph Primmer slapped a coin cell on a piezo and rickrolled a university

Right now, we’re looking at the entries to the Coin Cell Challenge, and there are some real gems here. Did you know the Rickroll Throwie maddeningly distributed around the dorms at Cornell is an entry? Yes, with just a coin cell, an ATtiny85, and a piezo, you can rickroll people for an entire year.

The top twenty projects for this contest will each receive $100 in Tindie credit to pick up some fancy kits and cool gear. The three top winners will each receive a $500 cash prize. We’re looking for three things specifically — a Lifetime Award that keeps a project going longest, a Supernova Award that drains a coin cell in the blink of an eye, and a Heavy Lifting Award that demonstrates what shouldn’t be possible with a simple coin cell.

This is your last weekend to submit a project, and the contest ends Monday afternoon, Pacific time. Enter now!